Subject headings are terms or phrases created by librarians to help you find everything there is about your topic in a database. The one difficulty with using subject headings is that you don't necessarily know what they are in advance, so you have to find out what the right subject headings are for your topic before you can do a really good search.
To find the subject headings for your topic, do a keyword search using words that make sense to you to describe your topic. Look through the results to see if anything looks like it's what you want. You may have to try a few different keywords before you get any good results. When you find something that you think is close to what you're looking for, open it up and look at the information the database gives about the item. You should see a list of subjects that are assigned to your item. These are the terms that you can use to do a more targeted search for items just on your topic. Be sure to specify in the search fields that you want to search subjects for these terms rather than keywords or titles.
Some examples of subject headings in philosophy
Bear in mind that some databases may not use the same subject headings. Library catalogs generally use the subject headings published by the Library of Congress. Databases like Philosopher's Index use different subject terms.
Knowledge, Theory of